Facile Graphene Oxide Modification Method via Hydroxyl-yne Click Reaction for Ultrasensitive and Ultrawide Monitoring Pressure Sensors

Enhancing the durability and functionality of existing materials through sustainable pathways and appropriate structural design represents a time- and cost-effective strategy for the development of advanced wearable devices. Herein, a facile graphene oxide (GO) modification method via the hydroxyl-yne click reaction is present for the first time. By the click coupling between propiolate esters and hydroxyl groups on GO under mild conditions, various functional molecules are successfully grafted onto the GO. The modified GO is characterized by FTIR, XRD, TGA, XPS, and contact angle, proving significantly improved dispersibility in various solvents. Besides the high efficiency, high selectivity, and mild reaction conditions, this method is highly practical and accessible, avoiding the need for prefunctionalizations, metals, or toxic reagents. Subsequently, a rGO-PDMS sponge-based piezoresistive sensor developed by modified GO-P2 as the sensitive material exhibits impressive performance: high sensitivity (335 kPa–1, 0.8–150 kPa), wide linear range (>500 kPa), low detection limit (0.8 kPa), and long-lasting durability (>5000 cycles). Various practical applications have been demonstrated, including body joint movement recognition and real-time monitoring of subtle movements. These results prove the practicality of the methodology and make the rGO-PDMS sponge-based pressure sensor a real candidate for a wide array of wearable applications.

After completion of the reaction, the target product propiolate esters 3 was purified by column chromatography or precipitation.
General procedure B (hydroxyl-yne click reaction) First, GO was dispersed in DMF (5 mg/mL) with the assistance of sonication.Then, the catalyst DABCO (20%) was added, followed by the dropwise addition of propiolate esters 3 in batches.The reaction was then stirred for 3 h at room temperature.The mixture was dialyzed by water or centrifuged to yield the modified GO-P.
Preparation of a PDMS Sponge: First, the PDMS main agent and curing agent were mixed at a mass ratio of 10:1, then the template cube sugar was immersed, vacuum pumped for 1 hour to remove air bubbles, and then the samples were cured at 80 °C for 3 h.The cube sugar was subsequently removed using hot water at 100 °C, and the resulting material was dried in a vacuum oven at 60 °C for 3 h to obtain PDMS sponge.

Preparation of a rGO-PDMS Sponge:
The prepared PDMS sponge was treated with plasma (oxygen, 10 SCCM, 1 min).Then immersed into an ethanol solution of APTES (5 mmol/mL) for 2 h at room temperature.The modified sponge was washed with DDW and dried in a vacuum oven at 60 °C for 3 h.The modified PDMS sponge was immersed in GO-P2/DDW dispersion (2 mg/mL) for 2 h.After the dip-coating process, the sponge was dried at 60 °C for 5 h to produce the GO-PDMS sponge.Subsequently, it underwent reduction using hydrazine hydrate vapor at 70 °C for 2 h, followed by washing with DDW and drying to yield the rGO-PDMS sponge.
Preparation of a rGO-PDMS Sponge based piezoresistive sensor: Gold interdigital electrodes were prepared on polyimide films by shadow-mask vaporization.The rGO-PDMS sponge sensing layer was placed on the electrode and two copper wires were connected by conductive silver paste.
Finally, after packaging with PE film, rGO-PDMS sponge based piezoresistive sensor was successfully prepared.The dimensions of the rGO-PDMS sponge are 10mm * 10mm * 2.5mm (length * width * thickness).The specific parameters of the interdigital electrode are illustrated in Figure S6.

Characterizations and Measurements:
The morphology and composition of the samples were characterized using scanning electron microscopy (SEM, Gemini SEM 500), X-ray diffraction spectroscopy (XRD, Bruker D8 ADVANCE), X-ray photoelectron spectroscopy (XPS, Thermo  Over the years, efforts have contributed to providing the most accepted mechanistic proposal for the Lewis-base (DABCO) -catalysed hydroxyl-yne click reaction. 1,2In this mechanism, the reaction is divided into three steps as shown in Scheme S2.In step 1, the catalyst (DABCO) nucleophilically attacks the triple bond to form a more basic zwitterion I.In step 2, zwitterion I reacts with GO to generate intermediates II and III (deprotonation of hydroxyl groups on GO).In      Water droplet images and water contact angle measurements were presented.The contact angle of GO was measured at 71.8°.After modification with the hydrophobic dodecyl propiolate, the contact angle of GO-P1 increased to 86°.On the other hand, after the modification with the hydrophilic mPEG ester, the contact angles of GO-P2 and GO-P3 decreased to 66.9° and 56.3° respectively.Notably, the contact angle difference between GO-P1 and GO-P3 in comparison to GO exceeded 10°, indicating a substantial alteration in the hydrophilic/hydrophobic nature of GO by graft modification.These results were consistent with the TGA and FTIR data, providing further confirmation of the successful hydroxyl-yne click modification.In this study, the bending tests were conducted using a homemade device.As illustrated in Figure S9, bending and resetting are achieved through the relative motion of two designed triangular prisms.Specifically, the sensor is fixed on a platform formed by two closely positioned prisms.The black prism is affixed to the substrate, while the red prism, driven by a motor, reciprocally rotates around the central axis, thereby achieving the bending and resetting of the device.Ideally, the rotation center here is a corner, resulting in a radius of curvature that approaches zero.The initial current (unloading current) is a crucial parameter that is not only associated with the sensor itself but also closely linked to the testing scenarios.For the pressure unloading-loading test, which approximates an ideal testing scenario primarily related to the intrinsic performance of the sensor, the typical actual unloading current is around 10 -10 .When calculating sensitivity, we use a value of 1*10 -9 as the unloading current to eliminate the influence of noise and ensure more reliable results.For bending and wearable applications tests, to eliminate noise interference (especially off-design motions of the sensor), it is common practice to fix the sensor onto a substrate.However, this fixation may subject the sensor to a certain level of initial pressure, resulting in an increase in the initial current.

Fisher
Scheme S1.Synthetic route for the functionalization of GO with a series of propiolates through a Scheme S2.Accepted mechanistic proposal for the Lewis-base (DABCO) -catalysed hydroxyl-

step 3 ,
III nucleophilically attacks intermediate II, undergoing nucleophilic substitution to yield the final product GO-P and the catalyst DABCO.The DABCO continues to participate in the catalytic cycle.It should be noted that step 1 is the fast step of the reaction.There is a competing reaction in step 2, as indicated by the dashed arrow, where zwitterion I might undergo nucleophilic substitution with the propiolates to generate by-product VI.This process can be effectively suppressed by reducing the concentration of the propiolates in the reaction system, which can be achieved by gradually adding them in batches, as outlined in General procedure B. Additionally, it should be noted that the by-product can be easily removed by centrifugation.

Figure S4 .
Figure S4.Dispersion of GO, hydrophobic modified GO-P1 and hydrophilic modified GO-P2 in

Figure S5 .
Figure S5.Optical images of the contact angles of GO, GO-P1, GO-P2 and GO-P3.

Figure S6 .
Figure S6.Parameter schematic of the interdigital electrode.

Figure S7 .
Figure S7.Schematic illustration of the sensing mechanism of the rGO-PDMS sponge based

Figure S8 .
Figure S8.The I-T curves of the sensor under 8 kPa pressure with different driving frequencies.

Figure S9 .
Figure S9.Photographs and schematics of a homemade bending test device: (a) and (c) bending,

Figure S10 .
Figure S10.Practical applications of the rGO-PDMS sponge based piezoresistive sensor for

Table S1 .
Comparison of this work with other reported flexible pressure sensors

Table S2 .
Unloading current of the sensors for different test scenarios.